A printed wiring board has been produced as following.
First, a thin copper foil for surface circuit formation is placed on a surface of an electric insulating substrate comprising a glass-epoxy resin or a glass-polyimide resin, etc., and then, it is heated under pressure to prepare a copper clad laminated board.
Subsequently, on this copper clad laminated board, through-holes are provided and plated in order, and by carrying out an etching treatment on a copper foil located on a surface of the copper clad laminated board, a wiring pattern is formed equipped with a desired line width and a desired line pitch. Finally, formation of solder resist or other finishing treatment is carried out.
To a copper foil to be used in this stage, a surface side which is to be attached to a substrate by heating under pressure is subjected to a surface-roughening treatment, so that it can exhibit an anchoring effect against the substrate with this rough surface, thereby enhancing a bond strength between the substrate and the copper foil to ensure reliability as a printed wiring board.
More recently, there has been used a copper foil with a resin in which a matte side of a copper foil has been coated in advance with an adhesive resin such as an epoxy resin and then, the adhesive resin is made an insulating resin layer in a semi-cured state (B stage). And a printed wiring board, especially a build-up wiring board has been prepared by attaching a side of the insulating layer onto a substrate by laminating press under heating.
Further, in accordance with a higher integration of the electronic parts of various kinds, a wiring pattern is also required to be of high density in those build-up wiring board, and there arise a need for printed wiring board with a wiring pattern comprising wirings of fine line width and line pitch, so called a fine pattern. For example, in case of a printed wiring board to be used for a semiconductor packaging, there has been desired a printed wiring board having a high density extra-fine wiring with a line width and line pitch of around 30 μm, respectively.
If a thick copper foil is used as a copper foil for forming a printed wiring, a time required for etching through the copper foil to the surface of the substrate is prolonged. As a result, the formed wiring pattern tends to loose a verticality at the sidewall of the wiring, decreasing an etching factor (Ef) represented by the following formula:Ef=2H/(B−T)                wherein H represents a thickness of a copper foil, B represents a bottom width of a formed wiring pattern and T represents a top width of the formed wiring pattern.        
Such a problem is not so serious when a line width of a wiring in a formed wiring pattern is wide. However, when a wiring pattern has a narrow line width, it can be resulted in open circuit.
On the other hand, in case of thinning a relatively thin copper foil such as a usual copper foil having 9 μm thickness or 12 μm thickness to a copper foil with a thickness of 3 to 5 μm by half etching, it is possible, in fact, to make the Ef value large. However, to ensure contact strength with the substrate, a surface of the copper foil contacting with the substrate is a roughened surface usually having a roughness Rz of 5 to 6 μm or so. Copper nodules of this roughened surface are pushed into the substrate, and therefore, longer time is required for etching treatment to remove those copper nodules by etching. Here, the surface roughness Rz means Rz regulated by the definition of “5.1 Ten-points average roughness” of “Definition and indication of surface roughness” mentioned in JIS-B-0601 (1994).
If the copper nodules which have been pushed into the substrate were not removed completely, they become residual copper, causing an insulating malfunction when line pitches in a wiring pattern are narrow.
Therefore, in a process of removing by etching the copper nodules which have been pushed into the substrate, etching of sidewalls of the already formed wiring pattern are progressed at the same time, resulting a lower Ef value.
In addition, a relatively thin copper foil having a thickness of 9 μm or 12 μm has a low mechanical strength, thereby easily causing a wrinkle or fold in a production process of a printed wiring board, and sometimes resulting in a breakage of the copper foil. Therefore, a problem arises such that the most careful attention must be paid in handling thereof.
As is described above, it is practically very difficult to produce a printed wiring board on which a fine wiring pattern is formed, whose Ef value is large, and at the same time, whose contact strength with a substrate is also high. Particularly, it is virtually impossible to form a wiring pattern of high density and extra-fine wiring with a line pitch and line width of around 30 μm, using a commercially available copper foil. And it is a fact that development of such a copper foil to enable production of a printed board with a high density extra-fine wiring pattern is earnestly desired.
A copper foil to be used in such a fine pattern preferably has a thickness of 9 μm or less, particularly suitably of 5 μm or less.
As an ultra-thin copper foil to be used for such a fine pattern, there has been proposed a complex foil in which an ultra-thin copper foil is directly electrodeposited on one surface of a carrier foil, interposed by a release layer (Japanese Patent Publication No. 16329/1978), and also a composite foil in which a chromate coating layer, a copper-nickel alloy layer and an ultra-thin copper foil layer are provided on a matte side of a carrier copper foil having a rough surface (Japanese Patent Publication No. 18401/1996).
Also, the present applicant has previously filed an application directed to a copper foil which is an ultra-thin copper foil with a carrier, comprising a copper foil with a surface roughness (Rz) of 1.5 μm or less as a carrier, and on the surface thereof, comprising a release layer and a copper electroplating layer, laminated in this order, and characterized in that the outermost surface of the copper electroplating layer is subjected to roughening (Japanese Provisional Patent Publication No. 269637/2000), and an ultra-thin copper foil with a carrier comprising a copper foil as a carrier, and on the surface thereof, comprising a release layer and a copper electroplating layer laminated in this order, and characterized in that the carrier foil and the copper electroplating layer are adhered more strongly in a proximate parts of the right and left edge than in a center part, and that the outermost surface of the copper electroplating layer is subjected to roughening (Japanese Provisional Patent Publication No. 331537/2000).
Applied Example of these ultra-thin copper foils with a carrier is shown in FIG. 1. The ultra-thin copper foil with a carrier, in which a release layer 2 and a copper electroplating layer 4 are formed in this order on one surface of a foil 1 as an carrier (hereinafter referred to as “a carrier foil”), has the outermost surface 4a of the copper electroplating layer surface-roughened. This roughened surface 4a is placed on a glass epoxy substrate (not shown in the figure), the whole material is pressed under heat for bonding, and then, the carrier foil 1 is peeled off and removed to expose a contacting surface of the copper electroplating layer with the carrier foil, on which a predetermined wiring pattern is formed.
The carrier foil 1 functions as a reinforcing material (carrier) for back-up until the above-mentioned thin copper electroplating layer 4 and the substrate are adhered. Moreover, the release layer 2 is a layer provided for promoting a peeling in separating the above-mentioned copper electroplating layer 4 and the carrier foil. It can be removed together with the carrier foil when the carrier foil is removed by peeling, and therefore, the carrier foil can be removed completely and easily.
On the other hand, to the copper electroplating layer 4 adhered to the glass epoxy substrate, through-holes are provided and plated in this order. And then, the copper foil located on the surface of the copper clad laminated board is subjected to an etching treatment to form a wiring pattern equipped with a desired line width and a desired line pitch. Finally, formation of a solder resist and other finishing treatment are carried out.
The ultra-thin copper foil with a carrier of this type enables formation of a fine pattern. In addition, it has a reputation that it is especially suitable for production of a wiring board of build-up type, due to its excellent handling property upon operation. However, on the other hand, there has been actualized problems as below.
(1) In case of using a heat resistant glass epoxy resin laminated board such as FR-4 grade, temperature at heat pressure is around 170° C., therefore, it is unlikely that the carrier foil cannot be peeled off after a copper foil with a carrier is laminated on the resin substrate. However, in case of using a higher heat resistant resin, especially using a polyimide resin for a substrate, temperature employed for a process exceeds 300° C. in either case of a casting method and a laminating press method. Therefore, when a chromium plating layer is used as a release layer as shown in Japanese Patent Publication No. 16329/1978, chromium is dispersed in a copper layer as a carrier causing adhesion of the carrier foil and the ultra-thin copper foil, making release infeasible.
A mechanism of this phenomenon that release becomes impossible can be considered as follows.
Diffusion of chromium to the ultra-thin copper side occurs by heating to high temperature, but is rather small as compared to diffusion of the same to the carrier foil. This is considered that the surface of the chromium layer is covered by a thin chromium hydrated oxide layer. To the contrary, diffusion of chromium occurs to the carrier foil side so that the surface portion of the carrier foil becomes a copper content-enriched copper-chromium alloy surface. The copper of the copper-chromium alloy binds to the copper of the ultra-thin copper foil by a metal bond whereby the carrier foil and the ultra-thin copper foil are considered to be adhered.
Also, the same is expected in case of using a chromate coating layer and a copper-nickel alloy layer as a release layer as shown in Japanese Patent Publication No. 18401/1996. In this case, too, especially copper in the copper-nickel alloy layer binds to the copper of the ultra-thin copper foil by a metal bond whereby the carrier foil and the ultra-thin copper foil are considered to be adhered.
(2) In formation of a via (via hole) of a build-up wiring board, a laser via method is mainly used due to its high performance, etc. Kinds of the preferable laser may include CO2 gas laser, Xe laser, excimer laser, YAG laser, Ar laser, etc.
Currently, it is CO2 gas laser that is mainly used. However, a wavelength of light that CO2 gas laser oscillates is in a region of infrared rays with a wavelength around 10,600 nm, and most of light and electromagnetic wave of this region is reflected at the surface of the copper foil. Accordingly, it is impossible to carry out laser drill directly from the surface of the copper foil. Therefore, a conformal mask method is employed in which part of the copper foil subjected to drilling process is removed by etching in advance followed by carrying out drilling process.
The conformal mask method requires a troublesome process in which an etching resist is coated on a copper electroplating layer 4 in FIG. 1, except for a part on which via holes are aimed to be provided, and after the copper foil is once removed by etching, the resin part is penetrated by combustion with CO2 gas laser. Accordingly, if it becomes possible to carry out drilling process directly through the copper foil by CO2 gas laser, the drilling process can be simplified.
An object of the present invention is to provide an ultra-thin copper foil with a carrier to be used in a production of a printed wiring board for a fine pattern, that resolves the above-mentioned two problems. That is, the present invention provides an ultra-thin copper foil with a carrier, which facilitates peeling of the carrier foil from a copper clad laminated board prepared by casting or hot-pressing at a high temperature and which enables direct drill by CO2 gas laser from the surface of the ultra-thin copper foil.